The mammalian ear contains two types of sensory receptor cells, inner hair cells (IHC) and outer hair cells (OHC). IHCs communicate auditory information to the brain, whereas OHCs function as local amplifiers to boost the input to IHCs. Without functioning OHCs, hearing threshold is severely degraded and selectivity to different sound frequencies is effectively eliminated. How OHCs boost the ear's response is one of the central issues of hearing science. There are two competing theories. In one, it is assumed that the input organelles of hair cells, so called stereocilia, are motile and feed energy back to the vibrating system in which they are embedded, thereby boosting the response. The other theory is based on the observation that OHCs change their length in response to changes in their electrical state. These length changes are mediated by a novel molecular motor, prestin, which we identified five years ago. Both theories have some experimental support and both have weaknesses. It is the principal aim of the present work to establish the mode of amplification by OHCs and thereby reconcile the two theories, The primary means of accomplishing this is to study the physiological behavior of mice engineered to express mutated forms of prestin. In other words, we examine response characteristics of animals that have the molecule prestin in their OHCs, but the molecule is non-functional. Thereby, these animals do not have motile OHCs and, consequently, one of the proposed means of amplification is absent, while the other is unaltered. Additional experiments are designed to examine the molecular nature of prestin, which may be the key molecule in mammalian hearing. Hearing loss and deafness, congenital or acquired during life, and especially prevalent in our aging population, represent one of the most common public health problems. A very significant fraction of these deficits involve deterioration or death of hair cells, especially OHCs. Our entire effort is focused on unreveling the properties of OHCs and their unique function in the hearing process.